Nitrogen Cycle Patterns During Forest Regrowth in an African Miombo woodland Landscape

Abstract

Tropical dry forests in eastern and southern Africa cover 2.5 × 106 km2, support wildlife habitat and livelihoods of more than 150 million people, and face threats from land use and climate change. To inform conservation, we need better understanding of ecosystem processes like nutrient cycling that regulate forest productivity and biomass accumulation. Here we report on patterns in nitrogen (N) cycling across a 100‐year forest regrowth chronosequence in the Tanzanian Miombo woodlands. Soil and vegetation indicators showed that low ecosystem N availability for trees persisted across young to mature forests. Ammonium dominated soil mineral N pools from 0‐ to 15‐cm depth. Laboratory‐measured soil N mineralization rates across 3‐ to 40‐year regrowth sites showed no significant trends and were lower than mature forest rates. Aboveground tree N pools increased at 6 to 7 kg N·ha−1·yr−1, accounting for the majority of ecosystem N accumulation. Foliar δ15N <0‰ in an N‐fixing canopy tree across all sites suggested that N fixation may contribute to ecosystem N cycle recovery. These results contrast N cycling in wetter tropical and Neotropical dry forests, where indicators of N scarcity diminish after several decades of regrowth. Our findings suggest that minimizing woody biomass removal, litter layer, and topsoil disturbance may be important to promote N cycle recovery and natural regeneration in Miombo woodlands. Higher rates of N mineralization in the wet season indicated a potential that climate change‐altered rainfall leading to extended dry periods may lower N availability through soil moisture‐dependent N mineralization pathways, particularly for mature forests.

Stable isotope signatures of soil nitrogen on an environmental–geomorphic gradient within the Congo Basin

“…The δ 15 N reported here for the Miombo woodland (2.68 ‰-4.32 ‰ between 0-20 cm, Fig. 2) falls within the same range as values reported by Wang et al (2013) from a moist woodland savanna in Zambia (2.6 ‰-4.8 ‰) and a mature Miombo woodland in Tanzania (3.5 ‰-3.8 ‰, Mayes et al, 2019), but they are lower compared to values reported from a dry forest in the Cerrado Savanna of Brazil (6.3 ‰-10.8 ‰, Bustamante et al, 2004).…”

“…On the other hand, fractionation during denitrification is variable with fractionation factors reported ranging from 0 ‰ to 33 ‰ (Högberg 1997). Nitrification and denitrification result in a gradual enrichment of soil δ 15 N. The process of N mineralization is believed to fractionate only marginally; however, in a system with high N mineralization rates, isotopic fractionation due to mineralization can be important (Nadelhoffer and Fry 1994). N 2 fixation depletes soil δ 15 N values, as it imports atmospheric N that has an isotopic value of 0 ‰ by default (Högberg, 1997).…”

Effects of Nutrient Deprivation on the Growth and Development of Tabebuia rosea Seedlings

“…The above study utilized Andosol substrates and the same experimental approach as the one presented here (nutrient deprivation). Despite the low concentration of N in the substrate used here, and taking into account that in volcanic ash-derived soils, N is limiting for plants given its low mineralization rate (Mayes et al, 2019), the deprivation of this nutrient did not affect T. rosea as drastically as the absence of other nutrients.…”